We examined central processing mechanism for speech perception and production using functional brain image such as positron emission tomography (PET) in subjects of hearing impairment and speech disorder as well as normal subjects. Postlingu ally deafened subjects indicated rather identical activation of auditory association areas with normal subjects. But, more activation was observed in cingulate gyms and supplementary motor area in deaf subjects. The brain imaging of prelingually deafened subjects showed quite different pattern. Most cases showed little activation in the auditory association areas by listening to speech sounds via cochlear implant. On the other hand, the auditory association areas were activated by lipreading in some prelingually deafened subjects. These results imply that the brain differentiation should differ according to verbal environment during speech acquisition period.As for vocalization, Broca* area, motor area, supplementary motor area, the cerebellum were important for output of speech. However, the auditory association areas were not activated during vocalization, which indicated that auditory feedback system was not needed to perform daily conversational vocalization. In contrast, when one* speech was distorted, the auditory association areas became to be activated, which implied the feedback system to repair vocalization. This situation is similar to that during speech acquisition period.In conclusion, we can observe various patterns of cortical processing mechanisms according to the situation of verbal environment.